1. Field of the Invention
The present invention relates to a single-mode optical fiber suitable for a transmission line in digital communications and a method of fabricating the same.
2. Related Background Art
Conventionally, in optical communication systems adopting a single-mode optical fiber (referred to as "SM optical fiber" hereinafter) as their transmission line, light of 1.3-.mu.m wavelength band or 1.55-.mu.m wavelength band has often been used. Recently, from the viewpoint of lowering transmission loss, the use of 1.55-.mu.m wavelength band light has been increasing. Such an SM optical fiber applied to transmission lines for 1.55-.mu.m wavelength band light (referred to as "1.55-.mu.m SM optical fiber" hereinafter) has been designed such that its wavelength dispersion (phenomenon in which pulse wave broadens due to the fact that velocity of propagation of light varies according to wavelength) becomes zero (a dispersion-shifted fiber whose zero dispersion wavelength is set 1.55 .mu.m). Currently, as such a dispersion-shifted fiber, optical fibers having a refractive index profile of a dual-shape type such as that disclosed in Japanese Patent Publication No. Hei 3-18161 or a segmented-core type disclosed in "Relation between Macrobending Losses and Cutoff Wavelength in Dispersion-Shifted Segmented-Core Fiber," Electronics Letter, No. 22, No. 11, p. 574, 1986 have been mainly used.
Also, in recent years, as long-distance transmission has become possible because of the advent of optical amplifier, in order to prevent the four-lightwave mixing, which is one of nonlinear optical effects, there has also been used an optical fiber in which the above-mentioned refractive index profile is modified so as to shift the zero dispersion wavelength from 1.55 .mu.m to the shorter or longer wavelength side. Here, the nonlinear optical effects refer to phenomenons in which signal light pulse is distorted in proportion to density or the like of optical intensity. They become a factor restricting the transmission rate.